The designing of tools, in particular rotatable tools such as drills for cutting or chip removing machining, involves a constant give-and-take between conflicting technical and economic constraints. Modern cutting tools are mostly composed of a basic body and one or more hard cutting inserts or wear bodies, which can be discarded after completed use. In such a way, long use of the comparatively expensive basic body is enabled (e.g., 10 to 30 insert exchanges). The plurality of requirements and desires that are made on the tools include good machining results in respect of precision, surface quality, repeatability and speed; low costs of the manufacture of the basic body as well as the cutting inserts; great strength of the tool in order to withstand severe stresses and rapid feeding; good chip control; etc.
The need for compromise becomes particularly evident in the designing of drills of the type that makes use of replaceable cutting inserts, and that by those skilled in the art are denominated indexable insert drills. Indexable insert drills having large or medium-sized holing diameters give rise to delicate balancing as a consequence of the complicated character of the drilling operation in respect of, for instance, chip control, balance, chip removal capacity, and the fact that the speed of rotation in the different parts of the drill body decreases from a maximum value at the periphery toward zero in the center of the drill body. However, the difficulties become particularly serious when indexable insert drills for small holes, i.e., holes having a small diameter, are to be produced. For spatial reasons, neither the cutting inserts, nor the pockets of the drill body for the inserts, can be made unlimitedly small. In particular, if the cutting inserts could be made comparatively sturdy, this will be on the expense of the material that surrounds the insert pockets in the basic body, i.e., the latter would become weakened. Conversely, a decreasing quantity of material (cemented carbide) in the cutting insert weakens the insert. Moreover, in small indexable insert drills, there is a special limitation so far that the only way in which the individual cutting insert can be fixed in the appurtenant pocket is to secure the insert by a screw, which has to have the head thereof countersunk in the hole in the cutting insert. Among other things for reasons of strength, the screw has to have a certain smallest diameter (e.g.>1.8 mm), which in turn means that the width of the cutting insert cannot be less than a certain minimum measure. Furthermore, the center insert should be protected inside a radially outer border, which during the drilling operation fills out the space between the center insert and the hole wall, in order to obviate the risk of chip stopping between the center insert and the hole wall.
In practice, the difficulties and limitations pointed out above have meant that previously known indexable insert drills of the type mentioned initially have not been possible to be made with diameters smaller than 12.7 mm. In the patent literature, thick and medium thick indexable insert drills having two-fold or four-fold indexable center inserts are disclosed in, for instance, U.S. Pat. No. 6,527,486, and International Patent Application Publication Nos. WO 03/099494 and WO 03/099495.
In this connection, the lack of space in a thin drill body is most problematic for the center insert and the pocket thereof, but less for the peripheral cutting insert and the pocket thereof. Thus, the peripheral pocket not only may, but should open in the envelope surface of the drill body. However, the center pocket should open only in the front end surface of the drill body and be situated inside a protecting border adjacent to the envelope surface in order to avoid chip stopping between the center insert and the hole wall generated.
An object of the invention to provide design conditions for the manufacture of a reliably operating indexable insert drill having a diminutive holing diameter, e.g., diameters less than 12.7 mm, without lowering the strength and service life of the cutting inserts, the stiffness and strength of the drill body, the balance of the drill, or the dimensions of the tightening screws.
Theoretically, the fundamental problem to provide an indexable insert drill for small holes could be solved by simply making the center insert thinner and locating the insert parallel to the center axis of the drill body. In such a way, a resistant protection border could be retained outside the center pocket. Because not only two cutting inserts, but also surrounding material in the drill body, having to be accommodated within a space of approx. 12 mm (or less), such a cutting insert would, however, become too weak, in particular in the thin portions that surround the screw hole. Another alternative would be to use a sufficiently wide and strong cutting insert and reduce the material (usually steel) that is present in the protection border, as well as in the partition wall that separates the two pockets. However, this alternative cannot be realized in practice, because such a material reduction would jeopardize the stiffness and the strength of the front end of the drill body. The last-mentioned alternative would be non-expedient even if the center pocket would be tilted in relation to the center axis, because then the protection border would become too thin or partly cut away.